This study investigates genetic control of blood pressure and possible biochemical mediators of the hemodynamic response to stress. The study utilizes monozygotic or dizygotic normal college age male twins. They are hospitalized on the Clinical Research Center for 5 days to determine the response of blood pressure, the renin-aldosterone system, urinary and plasma norepinephrine to sodium loading and sodium depletion. At another time, subjects are restudied to assess the hemodynamic and psychophysiologic response to standardized stress procedures including isometric handgrip, cold pressor test, and the mirror drawing test. These studies will aid in determining which of these factors may be genetically controlled and, therefore, useful in the study of human hypertension in families.